Satellite-retrieved vertical profiles of methane over the Indian region: impact of synoptic­scale meteorology

Abstract

ABSTRACT The altitude distribution of methane (CH4) is the least addressed topic in the greenhouse gas assessment over the Indian region. In the absence of the in-situ measurements, the satellite-based retrievals of the vertical distribution of CH4 using Atmospheric Infrared Sounder (AIRS) measurements during the period 2003–2015 were made use in this study for the first time to understand the 3D distribution (latitude-longitude-altitude) of CH4 over Indian region. Significant regional and seasonal variations are observed in the vertical distribution of CH4, even though it is a long-lived greenhouse gas and known to be well-mixed. Over most of the regions, the highest mixing ratio is observed during post-monsoon months and minimum in the pre-monsoon/monsoon season. The presence of a ‘high altitude peak’ in CH4 (around 1880 ppbv) around 300 hPa–250 hPa was noted in post-monsoon which is caused by the monsoon-associated convective updrafts and the anti-cyclonic system. The vertical profiles show seasonal variations which are region as well as altitude-dependent. Over the oceanic region, the highest seasonal amplitude of CH4 mixing ratio was observed over North–Arabian Sea due to the proximity of the source rich land regions. During the winter and pre-monsoon months, the latitudinal differences are absent throughout the troposphere. A consistent increasing trend in CH4, ranging from 1 ppbv year–1 to 6 ppbv year–1 is seen at all the tropospheric altitudes, with faster growth rates at higher altitudes, maximizing at 300 hPa–150 hPa. An approximate estimate of direct forcing due to CH4 lies in the range 0.80 W m–2–0.83 W m–2. The paper also presents a comparison of the in-situ measured upper tropospheric CH4 mixing ratio from CARIBIC (Civil Aircraft for the Regular Investigation of the atmosphere Based on an Instrument Container) flight data and AIRS retrievals.